1. Field of the Invention
This invention relates to an automatic transmission control system.
2. Description of the Prior Art
An automatic transmission is provided with a plurality of frictional members for selecting gear speeds, and the transmission is shifted into a desired gear speed by suitably controlling the frictional members. In some types of automatic transmissions, one of a pair of frictional members is caused to engage and the other is caused to disengage when the transmission is shifted down. For example, when the transmission is in third in D (drive) range (first to fourth), a front clutch is engaged and a second brake is disengaged, and when the transmission is shifted down from third to second, the front clutch is caused to disengage and the second brake is caused to engage. In the hydraulic pressure control system for the front clutch and the second brake, the front clutch and the second brake generally have the hydraulic pressure supply system and the hydraulic pressure release system in common. That is, hydraulic pressure for engaging the front clutch is used for disengaging the second brake, and the drain passage for disengaging the front clutch is used as the drain passage for draining second brake disengaging hydraulic pressure and engaging the second brake.
In the automatic transmission in which downshift is controlled in such a manner, shift shock can occur if the frictional member for selecting the gear speeds is not operated in time. For example, when the transmission is shifted down from third to second in D range, the turbine rpm of the torque converter is increased in response to disengagement of the front clutch from N.sub.3, the converter rpm when the transmission is in third, to N.sub.2, the converter rpm when the transmission is in second, by the rpm corresponding to the difference between the gear ratios in third and second. Accordingly, if engagement of the second brake is just completed at the time the turbine rpm reaches the rpm N.sub.2, the downshift can be smoothly effected and less shift shock is generated. On the other hand, the second brake is engaged in second and disengaged in first, and accordingly, the second brake must be caused to engage when the transmission is shifted up from first to second. Shift shock is apt to occur also when the transmission is shifted up from first to second, and it has been known, as disclosed in Japanese Unexamined Patent Publication No. 59-183160, to cause the second brake to smoothly engage in order to prevent occurrence of shift shock by providing an accumulator on the hydraulic pressure supply side of the actuator of the second brake and by gently supplying hydraulic pressure to the accumulator through a throttling valve. However, if the second brake is caused to engage by gently supplying hydraulic pressure to the accumulator through the throttling valve when the transmission is shifted down from third to second, engagement of the second brake delays to elongate the time the transmission is in neutral, and the turbine rpm exceeds the rpm N.sub.2 at the end of the downshift, thereby causing shift shock.